The goal of this project is to obtain a detailed physical understanding of how water, ions, and nonelectrolytes cross epithelia syecialized for fluid transport. The focus is on transport and permeability properties of cell membranes and cell junctions; and on understanding permeation in terms of structures and forces at the molecular level. The problems that will be investigated include the following: 1. Mammalian urinary bladder has been neglected by membrane physiologists but seems likely to prove of considerable physiological and clinical interest because of its recently rediscovered ion pumps, its extreme "tightness" to permeating ions, and its possible value as a model system for understanding effects of hormones and diuretics on the mammalian kidney. We shall study the nature of the ion pumps of mammalian urinary bladder, their possible physiological significance, the route and mechanisms of passive ion permeation, and modification of these properties by hormones, diuretics, and stretch. 2. Nitrogenous cations, because of their diversity of sizes and shapes, promise to provide sensitive probes of cation-selective sites in membranes. Permeabilities of numerous nitrogenous cations will be compared in several epithelia with "leaky" junctions, in order to construct selectivity isotherms and to recognize effects of field strength, hydrogen bonds, and steric restriction on permeation. 3. Factors determining rate barriers to nonelectrolyte permeation within lipid bilayer membranes and biological membranes will be measured. In particular, partition coefficients will be studied by radioactive tracers; solute concentration profiles within membranes, by nuclear magnetic resonance spectroscopy; and diffusion coefficients, partition coefficients, and interfacial barriers, by electron spin resonance spectroscopy.